1. Technical Field
This invention relates generally to electron beam projection systems and more particularly to an electron beam projection system which shifts the electron optical axis so as to be coincident with the deflected electron beam while eliminating rapidly varying magnetic fields and eddy currents in the target area.
2. Background Art
Electron beam projection systems are presently enjoying wide use in the semiconductor lithography and mask making fields. An electron beam projection system typically includes an electron beam source, a deflection system for deflecting the electron beam in a predetermined pattern and magnetic projection lenses for focusing the electron beam. The deflected and focused beam is directed to a target which may be, for example, a semiconductor substrate or a mask.
As the circuit density of integrated circuits increases, greater demands are placed on electron beam systems. More particularly, aberrations in the electron beam must be reduced so that accurate control of electron beam shape and position may be obtained.
A major advance toward elimination of electron beam projection system aberrations is described in U.S. Pat. No. 4,376,249 to Pfeiffer et al., which is assigned to the assignee of the present invention. In the Pfeiffer et al. patent, a variable axis electron beam projection system is described wherein the electron optical axis is shifted so as to be coincident with the deflected electron beam at all times. Shifting of the electron optical axis causes the electron beam to always land perpendicular to the target and eliminates lens aberrations caused by off-axis electron beams.
The variable axis electron beam system is a major advance toward eliminating off-axis aberrations in electron beam systems. However, once the major electron beam resolution limiting factors are eliminated, other factors which were previously of little importance now become performance limiting. One such factor is the production of eddy currents caused by the presence of conductive material in the areas of dynamic magnetic fields. The undesired magnetic fields produced by eddy currents and the resultant positional electron beam instabilities present serious problems to the system designer.
In the variable axis electron beam projection system described in the above noted Pfeiffer et al. patent, eddy current production in the electron beam target area has particularly become a problem. This may be seen by examining the configuration of the variable axis electron beam projection system. The system includes an electron beam source and a deflection means. A magnetic projection lens is employed for focusing the deflected beam at a target. The target lies below the lower pole piece of the magnetic projection lens. A first magnetic compensation yoke is positioned within the upper portion of the bore of the projection lens pole piece and a second magnetic compensation yoke is positioned within the lower portion of the bore of the projection lens pole piece. The first and second magnetic compensation yokes produce a magnetic field distribution which is proportional to the first derivative of the axial magnetic field strength distribution of the projection lens. This causes the electron optical axis of the projection lens to shift in conjunction with the deflection of the electron beam.
The above described variable axis electron beam projection system includes a lower compensation yoke positioned within the lower bore of the projection lens, adjacent the target. The rapidly changing field produced by the lower compensation yoke extends into the target area. This rapidly changing magnetic field can produce eddy currents in the target holder, target stepper table and other components containing conductive material, which are employed for supporting, aligning and moving the target. The eddy currents in turn produce stray magnetic fields which produce aberrations in the deflected electron beam.
Of course, eddy currents in the target area could be reduced by moving the target away from the lower pole piece. However, this would increase the aberrations in the overall electron beam system as the distance between the projection lens and the target is increased. Alternatively, eddy currents could be prevented by fabricating all the components in the target area of non-metallic materials. This is difficult, however, because of the rigidity, precision and durability required of the target area components. Accordingly, in order to further improve beam placement accuracy, means are required for eliminating eddy currents in the target area while still producing a compensation field which is proportional to the first derivative of the projection lens field.